1. Field of the Invention
The present invention relates to a display characteristic determining device for determining display characteristic of a display device such as a CRT. More particularly, the present invention relates to a display characteristic determining device to be used in a system capable of performing a color matching between a display device and a printing device.
2. Description of Related Art
Color of light emitted from a CRT and color printed by a color printer are different from each other due to the differences between their employing coloring mechanism and their using primary colors. A color calibration device is therefore required to calibrate colors on the CRT so that the colors appear the same as those on the printer. It is noted, however, that each type of CRT device has its own display characteristic. The display characteristic of each CRT device changes in time. Accordingly, in order to achieve proper color calibration, it is necessary to correctly know the display characteristic of the CRT device.
There is a CRT device of a type provided with an adjustment dial for allowing a user to adjust, into his/her desired degrees, both brightness and contrast, i.e., difference between dark and light portions. The display characteristic therefore varies also according to the user's adjustment operation. Accordingly, it is also necessary to know the present display characteristic in order to perform the color calibration.
The display characteristic of the display device depends on a relationship between a tone signal value inputted to the display device to display each primary color (red, green, and blue) and brightness actually obtained on the CRT device. This relationship will be referred to as "tone reproduction" hereinafter.
The tone reproduction can be represented by the following equation (1): EQU V={(L-BK)/(255-BK)}.sup..gamma. (1)
where V (0.ltoreq.V.ltoreq.1) is a value of brightness relative to a maximum brightness value which is normalized to one (1), L (0.ltoreq.L.ltoreq.255) is a value of a tone signal inputted to the CRT device for each of the R, G, and B components, BK (0.ltoreq.BK.ltoreq.255) is a black point, and .gamma. (.gamma.&gt;0) is a tone reproduction value. It is noted that V=0 . . . (2) where L&lt;BK.
FIGS. 1-4 show tone reproductions of various types of CRT devices. FIG. 1 shows a tone reproduction of a 19 inch display produced by Mackintosh Corporation. In the graph, points+indicate actually-measured values, and a solid line indicates values obtained through calculating the formula (1) where BK=16 and .gamma.=1.87. It is therefore apparent that the formula (1) can approximate the actual tone reproduction. FIGS. 2-4 show tone reproduction of a display of 2414 type produced by IBM Corporation when an attached adjustment dial is manipulated to change its tone reproduction. In FIG. 2, points+indicate values actually measured when the adjustment dial is manipulated to select a high contrast. In the graph, solid line indicates values obtained through calculating the formula (1) where BK=68 and .gamma.=2.02. In FIG. 3, points+indicate values actually measured when the adjustment dial is manipulated to select a middle contrast. A solid line indicates values obtained through calculating the formula (1) where BK=32 and .gamma.=1.54. In FIG. 4, points+indicate values actually obtained when the dial is manipulated to select a low contrast. A solid line indicates values obtained through calculating the formula (1) where BK=0 and .gamma.=1.38. It is apparent from the figure that even when the tone reproduction changes according to the display type and according to a user's adjustment, the tone reproduction can be approximated by the formula (1) with corresponding variables BK and .gamma.. It is therefore apparent that those values BK and .gamma. determine the tone reproduction of the CRT.
Various methods have been proposed to determine the tone reproduction without using an expensive detecting device. For example, one method has been proposed in U.S. Pat. No. 5,298,993.
Based on the method disclosed in the patent, it is conceivable to determine the tone reproduction in a manner as described below.
The CRT device is controlled to display two display parts A1 and A2 shown in FIG. 5. In the display part A1, two tone portions, obtained by tone signals L1 and L2, are arranged alternately where 0.ltoreq.L1,L2.ltoreq.255 and L1&lt;L2. It is assumed that the tone portion obtained by the tone signal L1 has brightness V1, and the tone portion obtained by the tone signal L2 has brightness V2. The other display part A2 is obtained only by tone signals L3 where L1&lt;L3&lt;L2.
It is further assumed that when tone signal values 0 and 255 are inputted to the CRT display device, the display device displays the most dark portion and the most light portion, respectively. Brightness V1 of the most dark portion is defined as zero (0), and brightness V2 of the most light portion is defined as one (1). That is, V1=0, V2=1. The tone reproduction is calculated through a procedure shown in FIG. 6.
First, in S101, the tone signal values L1 and L2 for the display part A1 are set to 0 and 255, respectively, and the CRT device is controlled with the tone signals L1 and L2 to display the display part A1. Because tone portions formed by the signals L1 and L2 are arranged alternately in the display part A, the display part A1 exhibits brightness of VA1 which is an average of the two brightness level V1 and V2. That is, EQU VA1=(V1+V2)/2=0.5
Then, in S102, a user adjusts the value of the tone signal L3 to be inputted to the CRT display device so that the brightness VA2 of the display part A2 will be perceived by the user as equal to the brightness VA1 of the display part A1. When the brightness of the display part A2 appears the same as that of the display part A1, the brightness values VA1 and VA2 satisfy the following equation: EQU VA2=VA1=0.5.
Then, in S103, the user sets the value VA1 as a brightness value obtained by the present tone signal value L3.
When it is assumed that BK=0, the formula (1) can be represented by the following formula (3): EQU V=(L/255).sup..gamma. (3)
This formula (3) can be also represented by the following formula (4): EQU .gamma.=log (V)/log (L/255) (4)
Accordingly, in S104, the values VA1 and L3 are substituted for the values V and L in the formula (4), and the following formula (5) is calculated: EQU .gamma.=log (VA1)/log (L3/255) (5)
Thus, the gamma value, which is one of the main variables representing the tone reproduction, can be determined through the user's visual examination of the display state of FIG. 5. The one main variable .gamma. can therefore be determined without using any auxiliary devices. The user can easily judge whether the display parts A1 and A2 have the same brightness.
Another U.S. Pat. No. 5,381,349 has proposed a method for determining another variable, i.e., black point BK, for representing the tone reproduction.
Based on the method of the patent, it is conceivable to determine a black point as described below with reference to FIG. 7.
First, in S110, the CRT display is controlled to display a test pattern shown in FIG. 8. The test pattern includes two identical parts A formed by tone signals of zero (0) and a part B formed by a variable tone signal LB. Then, the value of the tone signal LB is set to one (1) in S111, and the test pattern is again displayed in S112. In S113, the user judges whether or not he/she perceives any difference in brightness between the parts A and B. When the user perceives no difference in brightness between the parts A and B (No in S113), the value of the tone signal LB is incremented by one (1) in S115, and the process of S112 is again performed. When the user perceives some difference in brightness between the parts A and B ("Yes" in S113), a black point BK is set as a value obtained by subtracting a value of one (1) from the present tone signal value LB in S114. That is, BK=LB-1.
The above-described process provides a simple method capable of easily determining a black point range BK that can provide the same brightness as that of the most dark portion (black portion).
Thus, the two variables BK and .gamma., representing the tone reproduction, can be obtained easily.
In the above-described conceivable methods, however, the user has to perform various adjustment operations while visually observing the display state. The user has to perform the adjustment operations while judging whether two displayed portions exhibit the same brightness and while judging whether there appears any difference in brightness between two displayed portions. Thus, the user has to adjust brightness of the adjustment tone region (A2 of FIG. 5 and B of FIG. 8) while visually comparing the brightness of the adjustment tone region with the brightness of the reference tone region (A1 of FIG. 5 and A of FIG. 8). These adjustment operations are still troublesome.